Magic: The Gathering Commander will consist of five different 100-card singleton decks. To continue the tradition of oversized cards associated with multiplayer releases, each deck will also include three premium oversized legendary commander cards that can be used in lieu of the normal versions of the cards (which are also included in the deck).
The 51 new cards will be legal in Eternal formats (that's Vintage and Legacy), but they will not be legal in Standard, Extended, Modern or Block Constructed.